Opt out of trait bounds on type params for #[derive(Animate)]

This allows us to not generate trait bounds for field types anymore,
by excluding bounds for type parameters used only in fields annotated
with `#[animation(error)]`.

The syntax is `#[animation(no_bound(Foo, Bar))]` where `Foo` is a type
parameter for the type on which we derive things. Ideally this should
be an attribute on the type parameter but this isn't stable yet.

components/style/values/generics/basic_shape.rs

@@ -43,6 +43,7 @@ pub enum ShapeBox {
 
 /// A shape source, for some reference box.
 #[allow(missing_docs)]
+#[animate(no_bound(ImageOrUrl))]
 #[derive(Animate, Clone, Debug, MallocSizeOf, PartialEq, ToComputedValue, ToCss)]
 pub enum ShapeSource<BasicShape, ReferenceBox, ImageOrUrl> {
     #[animation(error)]